Flood Risk Assessments

Site-specific flood risk assessments for planning applications, from single extensions to housing developments, across London, Surrey, Sussex, Kent and Essex. Because we also design the drainage and SuDS, your flood risk assessment and drainage strategy are prepared together and tell the same story to the council.

Get a flood risk assessment quote or call 020 3835 2633

When do you need a flood risk assessment?
Your council will usually ask for one if your site is:
• In Flood Zone 2 or 3 on the Environment Agency’s flood map for planning
• In Flood Zone 1 but 1 hectare or larger
• In an area with critical drainage problems, or shown to be at risk of surface water, groundwater or reservoir flooding
• Identified in the council’s Strategic Flood Risk Assessment as at increased risk
It applies to extensions as well as new homes. For minor extensions in Flood Zones 2 and 3, a simpler assessment based on the Environment Agency’s standing advice is often enough. Read our guide to flood zones →

What’s included
• A review of all sources of flooding: rivers and the sea, surface water, groundwater, sewers and reservoirs
• Environment Agency flood data and the council’s Strategic Flood Risk Assessment
• An allowance for climate change, using the Environment Agency’s guidance for your area
• The sequential and exception tests, where they apply
• Mitigation measures, such as raised floor levels, flood resilient construction and safe access and escape
• How surface water from the development will be managed, with a SuDS drainage strategy where needed
• A clear report ready to submit with your planning application

Projects we assess
Extensions, new homes, conversions and changes of use, and small residential and mixed-use developments, on river, tidal and coastal sites as well as those at risk from surface water or groundwater.

Levels of assessment
We match the assessment to the site and the risk, so you don’t pay for more than your council needs:
• Screening: a quick check of whether flood risk is an issue for the site at all, and what the council will expect
• Scoping: identifying which sources of flooding matter and what data is available, often enough for minor development
• Detailed assessment: a full, site-specific analysis of flood levels, depths and mitigation where the risk is higher

Flood vulnerability
National planning guidance groups development by how vulnerable it is to flooding, and this decides what’s allowed in each flood zone:
• Highly vulnerable: including basement homes, which are not normally allowed in Flood Zone 3
• More vulnerable: homes, care homes, schools and hospitals
• Less vulnerable: shops, offices, restaurants and industrial units
• Water-compatible: such as docks, marinas and some outdoor recreation
Turning a shop or office into homes moves it into a more vulnerable class, which is why a change of use can need a flood risk assessment even with no building work.

Tidal and coastal sites
Many sites along the Thames, the Sussex coast and the Kent marshes are protected by flood defences. We look at what would happen if a defence was overtopped or breached, using Environment Agency data where it’s available, as councils will often ask for this.

Guidance we follow
Our assessments are prepared in line with national planning policy and Government guidance:
• National Planning Policy Framework
• Climate change
• Planning Practice Guidance: Flood risk and coastal change
• Flood risk assessment for planning applications
• Flood risk assessment: standing advice
• Flood risk assessment guidance for new development
• Your council’s Strategic Flood Risk Assessment and local plan policies

What we need to quote
• Site address or postcode
• Existing and proposed drawings, including floor levels if you have them
• Planning reference and any comments from the council or Environment Agency
• Any topographical survey

How it works
1. Send us the site address, your drawings and any planning comments.
2. We check the flood maps and confirm what level of assessment your council will expect, then give you a fixed quote.
3. We gather the flood data, carry out the assessment and design any mitigation and drainage.
4. You receive the report for your planning application, and we respond to any comments from the council, the Environment Agency or the Lead Local Flood Authority.

Common questions

How do I know which flood zone my site is in?
Check the Environment Agency’s flood map for planning on GOV.UK. Send us your address and we’ll check it for you as part of your quote.

What’s the difference between a flood risk assessment and a drainage strategy?
A flood risk assessment looks at whether the site could flood and how the development will stay safe. A drainage strategy shows how rainwater from the new buildings and hard surfaces will be managed so it doesn’t increase flooding elsewhere. Many councils ask for both, and we prepare them together.

What if the Environment Agency objects?
We review their comments, provide any extra information or revised mitigation, and respond on your behalf.

Can you help after planning?
Yes. We design the drainage, discharge pre-commencement drainage conditions, and handle sewer connections, adoptions, diversions and build-over agreements with the water company.

Do I need a flood risk assessment for an extension?
If the site is in Flood Zone 2 or 3, usually yes, but for a minor extension it’s normally a short assessment based on the Environment Agency’s standing advice. This typically means floor levels no lower than the existing house, or 300mm above the estimated flood level, plus flood resilient construction where needed.

Can I build a basement in a flood zone?
Basement bedrooms and self-contained basement flats count as highly vulnerable, and are not normally allowed in Flood Zone 3. Other basement uses may be possible with the right mitigation, which we can advise on.

What are the sequential and exception tests?
The sequential test asks whether the development could go on a site at lower flood risk. The exception test, where it applies, asks you to show the development has wider benefits and will be safe for its lifetime. They don’t usually apply to minor extensions or most changes of use. We tell you at the outset whether they apply.

Do I need a flood warning and evacuation plan?
Councils often ask for one for homes in Flood Zone 3 or where safe dry access isn’t possible. It sets out how occupants will receive warnings and leave safely, and we can prepare it with the assessment.

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Is my property at risk of flooding? →
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